In a cage, rolling elements are stored with clearances formed between the rolling elements and cage in order that the rolling elements rotating on their axes by the rotation of inner and outer rings can be prevented from touching each other. The cage can move three-dimensionally within a range permitted by clearances formed between the cage and inner or outer ring.
An ideally formed rolling bearing, in which inner and outer rings have the same inclinations and rolling elements have the same diameters, has a characteristic that its rotation is stable and the rolling elements are arranged evenly. Especially, a precision bearing for use in a machine tool spindle or the like is high in part precision and can be incorporated into the machine with high precision. Therefore, in operation, the rolling elements are easy to be arranged evenly. On the other hand, in order to operate a rolling bearing stably when an unbalanced load applied to the rolling bearing varies, the even arrangement of the rolling elements is necessary; and thus, the pockets of the cage are basically arranged evenly.
The cage is rotated by drive power from the rolling elements revolving (along the circumference of the bearing) by the rotation of inner and outer rings. Also, a cage includes: a rolling element guide type cage the revolution of which is guided while the radial-direction movement thereof is restricted by rolling elements through the pockets of the cage; and, a raceway ring guide type cage the revolution of which is guided while the radial-direction movement thereof is restricted by the outside diameter of the inner ring or by the inside diameter of the outer ring through the inside diameter or outside diameter of the cage.
FIGS. 8A and 8B show an example of a rolling bearing 82 using a cage of a conventional inner ring guide type. FIG. 8A is a cross section view of the rolling bearing 82, and FIG. 8B is a longitudinal section view of the rolling bearing 82. As shown in FIGS. 8A and 8B, in an inner ring guide type cage 81, in order that its radial-direction movement can be restricted by an inner ring 84 and its revolution can be guided smoothly, there is formed a minute clearance 85 between the inside diameter of the cage 81 and the outside diameter of the inner ring 84. Although not shown, in an outer ring guide type cage as well, similarly, the radial-direction movement is restricted by the outer ring and a minute clearance is formed between the outside diameter of the cage and the inside diameter of the outer ring.
In a rolling element guide type cage, in order that the radial-direction movement is restricted by rolling elements through the pockets of the cage and the revolution can be guided smoothly, there are formed minute clearances respectively between the pockets and rolling elements stored in the pockets. In this case, the amount of restriction of the radial-direction movement of the cage is determined in consideration of the minute clearances between the pockets and rolling elements, the positions of the respective pockets, the positions of the rolling elements in the pockets and the like.
The rolling element guide type cage has a plurality of guide portions, that is, the plurality of pockets, whereas the raceway ring guide type cage has one guide portion, that is, the inside diameter or outside diameter. Therefore, since the raceway ring guide type cage can restrict the radial-direction movement of the cage with higher precision, the cage is hard to swing greatly within the bearing and thus is used often in a bearing required of high precision in high loads and in high-speed rotation.
Since the cage 81 has clearances between the pockets 83 and rolling elements 86, when the cage 81 revolves ideally and the rolling elements are arranged evenly, the cage 81 is not restricted by the rolling elements 86. However, when the cage 81 loses balance due to external force such as gravity and friction and thus the rolling elements 86 are caused to move relative to the bearing 82, there is a possibility that the cage 81 can rotate while swinging. Thus, in order to prevent the swinging motion of the cage 81, there is provided a technology which arranges the rolling elements 86 unevenly to thereby restrict the cage 81 (for example, see the patent document 1).
In the patent document 1, in order to restrict the cage, the center of one pocket is slightly shifted from a pitch circle connecting the centers of other pockets, and the revolving movements of the rolling elements are out of phase so as to prevent the rolling elements from being arranged evenly in the pockets disposed at regular intervals in the circumferential direction.
There is also proposed a rolling element guide type cage including: a restricting pocket portion having an axial-direction pocket clearance formed narrow; and, a rolling element guide pocket portion with a spherical surface having a slightly larger curvature than a ball used as the rolling element (for example, see the patent document 2). In the patent document 2, the contact of the ball serving as the rolling element with the inner ring and outer ring is prevented by forming an end portion in the spherical surface of the rolling element guide pocket portion, and the axial-direction movement of the cage is restricted by the restricting pocket portion, while the circumferential-direction movement of the cage is restricted by the rolling element guide pocket portion.